Method and improved device for measuring and locating a tooth apex

ABSTRACT

A process and a device for measuring and locating the apex of a root canal of a tooth, by measuring, between a rasp forming an electrode inserted in the root canal and a second electrode disposed in the mouth, the variations of time constants of resistances and capacitances encountered in the canal, includes applying to the terminals of the electrodes a continuous current signal or a square signal of current of predetermined frequency for repeating the measurements and carrying out at the terminals of the electrodes at least two measurements for a given alternation so that a radiometric computation of the measurements can be obtained which result represents a distance separating the distal end of the rasp from the apex.

BACKGROUND OF THE INVENTION

The present invention relates to an improved process and device forlocating the apex of a tooth.

More exactly, the invention provides a process and device for measuringin real time the distance between the distal end of an electrodeinserted in the root canal of a tooth and the apex of said canal, thismeasurement being insensitive to anything other than the pulp, in saidcanal, of blood, pus, debris, water or antiseptics such as hydrogenperoxide, sodium chloride or sodium hypochlorite.

DESCRIPTION OF THE RELATED ART

The location of the apex or the apical constriction is important in theprocedure of treating the dental canal because the success of the latterdepends on the total removal of the pulp tissue.

To this end, the dentist uses a metallic endocanal rasp which slides inthe root and, by its movement of rotation and back-and-forth movement,it scrapes the walls of the canal so as to depulp it and to descend moreand more deeply into the root, and this without extending beyond theapical constriction, which can cause trouble for the patient and lead toan abscess.

It is therefore important to be able to locate precisely the apex of theroot to be cleaned.

For this purpose, several processes have been used.

The process most used at present consists in inserting a rasp into thecanal by successive approaches and monitoring with a succession of x-raysamples to place the rasp in position at the apex.

Then the dentist adjusts the abutment of the rasp against a cuspid scaleto determine the depth of the canal.

This technique is inadequate and undesirable because of the dose ofradiation to which the patient is subjected or because of curves in thecanal or a lateral apex, which is a source of errors of reading and ofanalysis of the samples.

Another process (U.S. Pat. No. 5,049,069) based on the discovery thatthe measurement of electrical impedance between an electrode placed atthe apex and a reference electrode placed in the mouth of the patient,gives a constant value no matter what the apex and no matter what thepatient. It has been established that between the apex and a point alongthe root canal, the impedance was proportional to the distance betweensaid point and the apex over several millimeters.

These processes, based on this discovery, are not reliable, because themeasurement depends on variations of impedance at the level of thereference electrode disposed in the mouth of the patient.

They are adjusted for a measurement made in a sound pulp and they do notfunction in media such as hypochlorite, sodium chloride, blood, etc.because the impedance of the medium is modified.

Another more reliable process (French patent 93/13802) operates withcanal media, known in advance, using measurements of impedance betweenan electrode such as a rasp inserted in the dental canal and two otherreference electrodes connected to the lip of the patient, and permitseliminating errors due to variations of the reference contacts and henceto increase the reliability of the measurement. It can measure in bloodor in very conductive media such as hypochloride, by creating a stagedoffset of origin of the resistivity of the medium.

Another process (U.S. Pat. Nos. 5,080,586 and 5,112,224) uses twoimpedance measurements at two different frequencies, for example 1 khzand 5 khz, between an electrode such as a rasp inserted in the rootcanal and an electrode abutting the oral mucosa, the impedances nothaving the same frequency response upon approaching the apex. There iscarried out a substraction of the two measurements. This difference inapproaching the apex will give the measurement. This process provides amanual or automatic zero reset of the output of the subtractor, by abutton or a detection, when the rasp has been inserted in the canal,acting on a memorized offset of one of the inputs of the subtractor, andpermits determining the movement of a zero reset at the output. Adifference between the impedances having frequent respective frequencyresponses discloses at a specified value the position of the apicalopening (FR 2.668.701-A1).

This process requires a calibration for each canal to be measured, whichis hardly practical or is a source of errors in current usage.

Another process (U.S. Pat. No. 5,096,419) uses two impedancemeasurements at two different frequencies, for example 400 hz and 8 khz,between an electrode such as a rasp inserted in the root canal and anelectrode abutting on the oral mucosa, the impedances having not thesame response in frequency to the approach of the apex. There is carriedout a ratio of the two measurements to eliminate the variation of theimpedances due to the media in which the rasp is located in the canal.

This process gives errors when the medium becomes insulating such ashydrogen peroxide. Thus the responses of the two frequencies are nolonger entirely ratiometric.

SUMMARY OF THE INVENTION

The present invention provides a different measurement process, withoutcalibration, independent of the medium in the canal, and of highprecision.

The present invention provides a process for locating the apex of a rootcanal of a tooth and/or for measuring the canal distance by means of themeasurement of variations of time constants of theresistances/capacitances encountered, between an electrode such as adental rasp inserted in the root canal, and a second electrode disposedfor example on the buccal mucosa of the patient, characterized in that:

There is applied to the terminals of the electrodes a current or a givencontinuous voltage.

A square signal of predetermined frequency permits repetition of themeasurements in real time, creating positive and negative alternation.

After amplification, the origin of the measurements is fixed at thelevel obtained at 0 μS.

There are thus carried out two time measurements of the alternation.

It will be noted on FIGS. 5 and 6, that one measurement (A) developsvery little when the rasp is located between 5 mm and 1.5 mm, andrevolves more in the region of the apex, whilst measurement (B) developsin a substantially linear manner.

A ratiometric computation of the measurements (A) and (B) is thencarried out to eliminate the effect of perturbances such as the changeof medium, the change of diameter of the rasps;$M = {{\frac{A}{B} - {{K1}\quad {or}\quad M}} = {{K1} - \frac{B}{A}}}$

The squared measurements before computation of the ratio give a betterlinearity, thus the development of the signal to be measured isexponential.

The formulae operate for conductive media and very conductive media,they operate a bit less well in media that are hardly conductive, suchas hydrogen peroxide.

To be less sensitive to the less conductive media:

There is carried out in time three measurements alternately. A measure(A) and a measure (B) carried out as before and a measure (C) as shownin FIGS. 5 and 6 which develops in the portion between 5 mm and 0.5 mmand very little in the region of the apex.

There is carried out the computation$M = {\frac{A\quad*\quad C}{B\quad*\quad B} - {K1}}$

or the calculation$M = {{K1} - \frac{B\quad*\quad B}{A\quad*\quad C}}$

So as to attain a final result of the distance on the desired scale, thevalue M is multiplied by a suitable coefficient, which represents thedistance comprised between the distal end of the rasp and the apex.

For the preceding radiometric computation, K1 is a known constant andpermits obtaining location of the apex at a value located at zerovoltage for example.

If a measurement is made with negative alternation, there is applied acurrent or a voltage of opposite direction. The resulting M will beinverted.

The measurements will be measurements of voltage at the terminals of theelectrodes if the signal applied to the electrodes is a current.

The measurements will be measurements of current passing through theelectrodes if the signal applied to the electrodes is a voltage.

The measurements are carried out with positive and/or negativealternation.

The present invention also provides a device for practicing the processof the invention, comprising an electrode inserted in the root canal anda second electrode disposed in the mouth, characterized in that itcomprises moreover:

Means to apply a square signal to the terminals of the electrodes;

Measuring means to obtain at least two values of voltage for a givenalternance;

Means for computing and processing the measured values, to determine thedistance between the distal end of the electrode inserted in the canaland the apex of said canal;

Display means to display said distance thus determined.

Preferably, the means to apply a square signal are constituted by acurrent generator and the measurements carried out are measurements ofvoltage at the terminals of the electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an embodiment of a device for practicing theprocess of the invention.

FIG. 2 is a graph showing qualitatively the square voltage applied bythe sequencer.

FIG. 3 is a graph illustrating qualitatively the voltage at theterminals of the two electrodes.

FIG. 4 is a graph illustrating qualitatively, for positive alternants,the different measurements effected.

FIG. 5 is a graph illustrating qualitatively the variations noted fordifferent distances separating the measurement electrode from the apex.

FIG. 6 is a graph showing qualitatively the variation of two measurementpoints as a function of the distance to the apex.

It should be noted that the graph described above simply givesqualitative information as to the observed signals, and no quantitativeinformation whatsoever can be determined therefrom.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 there is shown in cross-section a tooth 10 comprising a rootcanal 12 terminating in an apex 14, the gums and the buccal tissuesbeing shown at 16.

An endodontic rasp 18, located in the canal 12, is an electrodeconnected to a flexible conductive wire 20 which is connected to theground. A second electrode 22, connected to a flexible conductive wire24, is opposed against the buccal mucosa 16 of the patient.

The wire 24 is connected to a module 26 itself connected to a sequencer28 which generates a square signal of predetermined frequency andamplitude.

The module 26 comprises a circuit adapted to limit the substantiallyconstant current intensity flowing through the tooth of the patient andto suppress the continuous component at the terminals of the electrodes18, 22. There can for example be a high ohmic resistance R in serieswith the condenser C of suitable capacitance.

The voltage at the terminals of the electrodes 18, 22 is amplified andmeasured by an amplifier 30 whose output is connected to ananalog-digital converter 32 synchronized by the sequencer 28.

The converter 32 is connected to three memories 36, 38, 40, respectivelysynchronized by the sequencer 28.

The memories 36, 38, 40 are connected to a computation unit 42.

The output of the unit 42 is connected to a digital and/or analogdisplay 44 and has comparators 46, 48, 50, 52, 54, 56, 58, respectivelyconnected to comparison memories 60, 62, 64, 66, 68, 70, 72.

The comparators 46, 48, 50, 52, 54 are connected to luminous signals 74,76, 78, 80, 82, whilst the comparators 56, 58 are connected to amodulator 84 connected to a sonic warning 86. The modulator 84 is alsoconnected to the output of the module 42, by a wire 88.

The computation unit 42 is also connected to comparators 94, 96, 98,respectively connected to memories 100, 102, 104. The comparator 94, 96are connected to a modulator 108 synchronized by the sequencer 28. Likethe modulator 84, the modulator 108 is connected to the output of thecomputation unit 42 by a wire 110. The output of the modulator 108 isconnected to a mode switch 112 synchronized by the sequencer 28.

The mode switch is connected to a mode selector 114 and to thecomparator 98. The output of the mode switch 112 is connected to arotational speed regulator 116 comprising means to adjust the speed 118,said regulator 116 piloting a rotatable electric motor 90 associatedwith a counter angle supporting a continuously rotating rasp 92.

The block 116 can also be an ultrasonic oscillator comprising poweradjustment means 118, said oscillator 116 controlling the oscillationsof the rasp 92.

The process of the present invention is described in detail below withrespect to the device described above, which represents a preferred butnon-limiting embodiment.

The sequence 28 generates a square signal of 50 hz such as that of FIG.2.

The voltage at the terminals of the electrodes 18, 22 is as in FIG. 3.

The signal obtained at the terminals of the electrodes 18, 22 is afunction of the variations of the time constants of theresistances/capacitances encountered in the canal, the latter has anamplitude which is connected to the medium in which the electrode 18 islocated. Between the conductive medium and a non-conductive medium, itis noted that the amplitude is less in a conductive medium.

The measurement is zeroed in front of the signal period at 0 μS which isthe beginning of alternation.

As shown in FIG. 4, which represents a positive alternation of thevoltage at the terminals of the electrodes 18, 22, there are carried outthree voltage measurements A, B, C, respectively at 100 μS, 2.5 mS, 10mS at the beginning of alternation. These measurements are stored inmemories 36, 38, 40.

It can be seen in FIGS. 5 and 6 that the measurement A develops verymuch less in the portion in which the rasp is located between 5 mm and1.5 mm, and develops in the region of the apex, whilst a measurement Cdevelops in the portion between 5 mm and 0.5 mm and very little in theregion of the apex. A measurement B located at 2.5 mS evolvessubstantially linearly.

The computation unit 42 carries out the following computation:$M = {\frac{A\quad*\quad C}{B\quad*\quad B} - {K1}}$

K1 is a constant obtained experimentally

K1 is a constant equal to (A*C)/(B*B) when the distal end of the rasp isat the apex of a tooth, which gives 0 for M.

The assembly remains ratiometric no matter what the medium encounteredin the canal and of the diameter of the rasp.

The linearity is sufficient over the range of measurement.

So as to obtain a final result of the distance on the desired scale, thevalue M is multiplied by a suitable coefficient, which represents thedistance comprised between the distal end of the rasp 18 and the apex14.

This final result is displayed on the module 44. The comparators 46, 48,50, 52, 54, after having compared this final result with referencevalues in respective memories 60, 62, 64, 66, 68, actuates as the casemay be the respective luminous signals 74, 76, 78, 80, 82.

The comparators 56, 58 compare the final result with reference values inrespective memories 70, 72, and actuate as the case may be the modulator84 so as to generate a suitable sonic signal. This permits monitoring ina visual and/or sonic manner the progressive approach of the rasp 18 tothe apex 14.

The mode selector 14 permits the dentist to select between differentmodes of operation of the ultrasonic rasp 92.

For example, an automatic operation mode permits applying vibrations tothe rasp as soon as it enters into contact with the moist medium of thecanal, by comparison with memory 104.

By a suitable choice of values stored in memory 100, 102, the modulator108 can also modify the cyclic ratio of the power applied by proceedingto short and instantaneous stoppages of the vibrations of the rasp 92.

For example, the vibration pattern of the rasp is constant when thislatter is more than 2 mm from the apex, corresponding to memory 100,then, from 2 mm to 1 mm, corresponding to memory 102, the regime ismodified by modification of the cyclic ratio, the vibrations decreasingprogressively until stopping when the rasp is 1 mm from the apex.

Another operating mode actuates the vibrations in forced operationbefore insertion of the rasp into the canal, which permits treating acalcified canal which otherwise would not permit operation in anautomatic mode.

Another mode actuates the vibrations solely between 2 mm and 1 mm fromthe apex to avoid false canals.

So as to limit errors of measurement due particularly to cavitation ofthe liquid about the rasp, the oscillator 116 can provide for short andmomentary stops of the vibrations applied to the rasp 92, in a mannersynchronized with the measurements that are carried out.

The handpiece 90 could be replaced by an endodontic counter angle, theblock 116 then becoming a rotation speed regulator with a standard speedof rotation 118, the comparators 96, 98 and reference values in theassociated memories 104, 102 giving rise to operation, stopping orreversal of direction of the motor of the counter angle driving a raspin continuous rotation serving as an electrode inserted in the rootcanal of the tooth.

The mode selector 114 permits the dentist to select between differentmodes of operation of the rasp rotated by the motor and its counterangle.

For example, an automatic operational mode permits applying rotation tothe rasp as soon as it enters into contact with the moist medium of thecanal, by comparison with the memory 104 and to stop it by comparisonwith the memory 102.

Another mode of operation actuates the rotation in forced operationbefore insertion of the rasp into the canal, which permits treating acalcified canal which cannot be treated by automatic operation.

Of course, the present invention is not limited to the embodimentsdescribed and illustrated, but covers on the contrary all modifications,particularly as to the elements relating to use of the measurementscarried out.

For example, the number of comparators and the reference values in theassociated memories can be modified as a function of the desired ends.

The measurements are carried out with positive alternation, but alsocould be carried out with negative alternation.

What is claimed is:
 1. A process for locating and measuring an apex of atooth root canal by measuring variations of time constants ofresistances and capacitances encountered between a measuring electrodeforming the first electrode inserted in the root canal and a secondelectrode disposed in contact with mouth tissues, comprising the stepsof: applying to the first and second electrodes a square wave signal ofa predetermined frequency; establishing a measurement baseline at alevel obtained at a beginning of an alternation of the square wavesignal; making two electrical measurements, A and B, in time during thealternation, computing a result M of a ratio of the two measurementswith a known constant K1; and converting the result M by a coefficientto a value representing the distance separating a distal end of thefirst electrode from the apex.
 2. Process according to claim 1, whereinthe result M is given by the formula:$M = {{\frac{A}{B} - {{K1}\quad {or}\quad M}} = {{K1} - {\frac{B}{A}.}}}$


3. Process according to claim 1, wherein the result M is given by theformula:$M = {{\frac{A\quad*\quad A}{B\quad*\quad B} - {{K1}\quad {or}\quad M}} = {{K1} - {\frac{B\quad*\quad B}{A\quad*\quad A}.}}}$


4. Process according to claim 1, further comprising the steps of: makinga third measurement C during the alternation, wherein the measurement Ais taken near the beginning of the alternation, the measurement C istaken remote from the beginning of alternation, and the measurement B istaken intermediate measurements A and C; and carrying out a ratiometriccomputation${M = {{\frac{A\quad*\quad C}{B\quad*\quad B} - {{K1}\quad {or}\quad M}} = {{K1} - \frac{B\quad*\quad B}{A\quad*\quad C}}}};$

in which K1 is a known constant.
 5. Process according to claim 1,wherein the two measurements are measurements of a voltage at terminalsof the first and second electrodes.
 6. Process according to claim 1,wherein the two measurements are measurements of current passing throughthe first and second electrodes.
 7. Process according to claim 1,wherein the two measurements are taken over a positive portion of thealternation.
 8. Process according to claim 1, wherein the twomeasurements are taken over a negative portion of the alternation. 9.Process according to claim 1, wherein the two measurements are takenover a portion of a positive or a negative alternation.
 10. Processaccording to claim 1, wherein the square wave signal applied has afrequency substantially equal to 50 hz, and the measurements A, B, and Crespectively are carried out at substantially 100 μS, 2.5 mS, and 10 mSfrom the beginning of the alternation.
 11. Device for locating andmeasuring an apex of a tooth root canal by measuring variations of timeconstants of resistances and capacitances encountered between ameasuring electrode forming the first electrode inserted in the rootcanal and a second electrode disposed in contact with mouth tissues,comprising: a first electrode adapted for insertion in the root canal; asecond electrode adapted for disposed in a patient's mouth; a squarewave signal source operatively connected to terminals of the first andsecond electrodes; a measuring means for obtaining at least two valuesof voltage, A and B, over one alternation of the square wave signal; acomputational module connected to the measuring means and determiningthe distance between the distal end of the first electrode when insertedin the canal and the apex of the canal by computing a result M of aratio of the two measurements A and B with a known constant K1 andconverting the result M by a coefficient to a value representing thedistance separating a distal end of the first electrode from the apex;and a display operatively connected to said computational module anddisplaying the determined distance.
 12. Device according to claim 11,wherein the square wave signal source comprises a constant currentgenerator and the measuring means is connected so that the measurementsof the voltage are carried out at terminals of the first and secondelectrodes.
 13. Device according to claim 11, wherein the displaycomprises a digital or an analog display.
 14. Device according to claim11, wherein the display comprises at least one comparator delivering aluminous or sonic signal when a reference value is reached.
 15. Deviceaccording to claim 11, further comprising: an ultrasonic rasp having avariable power and cyclic vibration ratio serving as another measuringelectrode for insertion in the root canal of the tooth; and pluralmemory locations for storage of reference values, the reference valuesserving as a comparison basis to associated measured distances. 16.Device according to claim 11, further comprising an electric motordriving in rotation a rasp serving as another measuring electrode forinsertion in the root canal of the tooth, plural memory locations forstorage of reference values, the reference values serving as acomparison basis to associated measured distances.